Registration endpoint accepts invalid email addresses

[elo-mate]

The POST /register endpoint does not validate email addresses. Users can submit forms with invalid emails like 'notanemail' or empty strings. The validateEmail function exists in src/utils/validation.js but is imported and never used in the route handler.

Steps to reproduce:

1. POST /register with body: { "email": "notanemail", "name": "Test" }
2. Response is 201 Created — should be 400 Bad Request

Expected: Return 400 with error message when email is invalid.
Actual: Accepts any string as email.